Prevent invalid encoding for files blowing up `Source.from_file`

[JonRowe]

From #354 an invalid encoding combination in a file can cause an error which stops RSpec from running and highlighting the correct cause:

Traceback (most recent call last):
	28: from bin/ruby_executable_hooks:24:in `<main>'
	27: from bin/ruby_executable_hooks:24:in `eval'
	26: from bin/rspec:23:in `<main>'
	25: from bin/rspec:23:in `load'
	24: from gems/rspec-core-3.8.0/exe/rspec:4:in `<top (required)>'
	23: from gems/rspec-core-3.8.0/lib/rspec/core/runner.rb:45:in `invoke'
	22: from gems/rspec-core-3.8.0/lib/rspec/core/runner.rb:71:in `run'
	21: from gems/rspec-core-3.8.0/lib/rspec/core/runner.rb:86:in `run'
	20: from gems/rspec-core-3.8.0/lib/rspec/core/runner.rb:98:in `setup'
	19: from gems/rspec-core-3.8.0/lib/rspec/core/configuration.rb:1558:in `load_spec_files'
	18: from gems/rspec-core-3.8.0/lib/rspec/core/configuration.rb:1558:in `each'
	17: from gems/rspec-core-3.8.0/lib/rspec/core/configuration.rb:1560:in `block in load_spec_files'
	16: from gems/rspec-core-3.8.0/lib/rspec/core/configuration.rb:2033:in `load_file_handling_errors'
	15: from gems/rspec-core-3.8.0/lib/rspec/core/configuration.rb:2037:in `rescue in load_file_handling_errors'
	14: from gems/rspec-core-3.8.0/lib/rspec/core/reporter.rb:161:in `notify_non_example_exception'
	13: from gems/rspec-core-3.8.0/lib/rspec/core/formatters/exception_presenter.rb:78:in `fully_formatted'
	12: from gems/rspec-core-3.8.0/lib/rspec/core/formatters/exception_presenter.rb:86:in `fully_formatted_lines'
	11: from gems/rspec-core-3.8.0/lib/rspec/core/formatters/exception_presenter.rb:240:in `formatted_message_and_backtrace'
	10: from gems/rspec-core-3.8.0/lib/rspec/core/formatters/exception_presenter.rb:34:in `colorized_message_lines'
	 9: from gems/rspec-core-3.8.0/lib/rspec/core/formatters/exception_presenter.rb:149:in `failure_lines'
	 8: from gems/rspec-core-3.8.0/lib/rspec/core/formatters/exception_presenter.rb:149:in `tap'
	 7: from gems/rspec-core-3.8.0/lib/rspec/core/formatters/exception_presenter.rb:150:in `block in failure_lines'
	 6: from gems/rspec-core-3.8.0/lib/rspec/core/formatters/exception_presenter.rb:163:in `failure_slash_error_lines'
	 5: from gems/rspec-core-3.8.0/lib/rspec/core/formatters/exception_presenter.rb:218:in `read_failed_lines'
	 4: from gems/rspec-core-3.8.0/lib/rspec/core/formatters/snippet_extractor.rb:30:in `extract_expression_lines_at'
	 3: from gems/rspec-core-3.8.0/lib/rspec/core/formatters/snippet_extractor.rb:18:in `source_from_file'
	 2: from gems/rspec-core-3.8.0/lib/rspec/core/world.rb:150:in `source_from_file'
	 1: from gems/rspec-support-3.8.0/lib/rspec/support/source.rb:12:in `from_file'
gems/rspec-support-3.8.0/lib/rspec/support/source.rb:12:in `read': U+043F from UTF-8 to ASCII-8BIT (Encoding::UndefinedConversionError)

After this PR:

An error occurred while loading ./spec/support/source_spec.broken_example.
Failure/Error: Unable to find matching line in ./spec/support/source_spec.broken_example

NameError:
  uninitialized constant UndeclaredModule