mplab.rb: Added /opt/microchip/mplabx because that is the default location used by the Linux installer.  Refactored the code for finding MPLAB X a little bit.
Contributing.md and Gemfile: Fixed typos.
Rakefile: Fixed the MPASM error handler so it really only catches errors from running MPASM and does not catch errors from constructing the command.
Rakefile: Changed the way we are invoking MPASMX to account for the fact that it does not actually return an error code in Linux.
spec/firmware: Fixed the capitalization of the included filenames so that firmware can compile on Linux.
mplab.rb: A few more adjustments to get the specs to run in Linux.  All specs are passing in Linux now except one.
Fixed path_retrieval_spec.rb so it can pass in Linux.  Now all the specs are passing.