Switch to libftdi-ruby from serialport.

Previously hacklet relied on a kernel driver to communicate with the
dongle. This change switches it to a cross platform library, which
allows hacklet to run on OSX as well as Windows.

Note: The Windows support is unconfirmed but should theoretically work.
I leave it as an exercise for a intrepid Windows user to confirm.
Minor README fixes

* Fix the issue link in the README.
* Reword the Linux permissions note.
Remove local reference from Gemfile.
Add libftdi to travis ci environment
Update libftdi-ruby to latest.
There is not 0.0.5 version of libftdi.
It turns out rts must also be set high.
Update spec to include rts
Update README with better Linux permission setup
Merge pull request #5 from mcolyer/libftdi

Switch to libftdi-ruby from serialport, fixes #2