Update gemspec

Changeset allows this gem to be installed with a more recent version of guard.

Moved the version constant to it's own file, previously parsing the gemspec required guard, which is where guard was being defined causing a catch-22 if guard was not already installed.

Made some changes in travis to get the build green.  May need to look further in to the configuration to get tests passing if support for rbx/jruby is necessary.