Always set the LC_ALL env var

This avoids mixlib shell from defaulting LC_ALL to 'C' if the caller didn't specifiy a value, in other words don't screw with LC_ALL. This fixes strange InvalidByteSequenceErrors when using this gem.
Added unit tests to cover LC_ALL environment var default
Fix failing Travis build which default LC_ALL to en_US.UTF-8:
Merge pull request #8 from daptiv/default-LC_ALL-to-parent-process

Default LC_ALL to parent process value