Updating get_file() to respect KERAS_HOME environment variable

[adrianjhpc]

This is a small feature update to enable the utils/data_utils.py get_file() function to respect the KERAS_HOME environment variable if set by a user. This would allow any datasets required by applications to be download to be cached in directories other than ~/.keras, which is especially useful for people running on systems where the HOME directory is not accessible from the place applications are being run.