Fix return type of callback function to rb_thread_call_with_gvl().

The return type is not used but caused a compiler warning:
  Function.c: In function 'callback_invoke':
  Function.c:479:9: warning: passing argument 1 of 'rb_thread_call_with_gvl' from incompatible pointer type [enabled by default]
  Function.c:102:14: note: expected 'void * (*)(void *)' but argument is of type 'VALUE (*)(void *)'

This addresses issue #297.
add correct RTLD constants at compile time
update Gemfile and .travis
Update .travis.yml
add bundler build byproducts to gitignore
update development dependencies
fix .travis.yml assumption
update MRI version
remove Rakefile cruft
add dev group to Gemfile
Merge pull request #329 from tduehr/rtld_mlock

Correct and update Gemfile. Additional testing on travis.
merge specs w/ JRuby and correct syntax to use rspec
correct spec task dependency
return of the magic encoding comment
fix require_relative
add rbx dependencies
add enum test to variadic spec
remove require_relative
Merge pull request #322 from tduehr/ffi_spec

merge specs w/ JRuby and correct syntax to use rspec
Merge pull request #309 from larskanis/fix-return-of-callback_with_gvl

Fix return type of callback function to rb_thread_call_with_gvl().