Fix Travis Build

[atz]

Dependencies aren't compatible anymore on half of the build environments, e.g.:

NoMethodError: undefined method `last_comment' for #<Rake::Application:0x00000001216348>
/home/travis/.rvm/gems/ruby-2.3.0/gems/rspec-core-2.99.2/lib/rspec/core/rake_task.rb:143:in `initialize'
/home/travis/build/mbklein/equivalent-xml/Rakefile:17:in `new'
/home/travis/build/mbklein/equivalent-xml/Rakefile:17:in `<top (required)>'
/home/travis/.rvm/gems/ruby-2.3.0/gems/rake-12.3.0/exe/rake:27:in `<top (required)>'
/home/travis/.rvm/gems/ruby-2.3.0/bin/ruby_executable_hooks:15:in `eval'
/home/travis/.rvm/gems/ruby-2.3.0/bin/ruby_executable_hooks:15:in `<main>'
(See full trace by running task with --trace)

https://travis-ci.org/mbklein/equivalent-xml/jobs/350014687#L1128

We need to specify a new enough version of rspec (safest option), or cap at a sufficiently old version of rake. StackOverflow explains, as usual.

Recommend updating rspec.

[atz]

For context, rspec v2.99 is from 2013, and its entire purpose was to provide all the deprecation warnings one would need to clear to get onto 3.x. I think it is safe to say we are done developing against rspec 2.x.