Add travis config for testing against multiple ruby versions

[PikachuEXE]

Also add missing dependencies

[PikachuEXE]

The build is green

[jeremyevans]

Thanks for the patch! I added another patch that modifies the travis configuration to be more like my other projects, and starts testing on JRuby, but it is good to add CI at this point.

[PikachuEXE]

I was trying to change eval to something else but failed
Got many things that cannot be fetched by module_eval
(ARGF.class, Complex::compatible)

Also some constants are undefined until file required (like Date)

[jeremyevans]

The things that cannot be fetched by eval issue could be handled by removing constant names from the module_names/*.txt file. Those files were produced by something like: ruby -e 'ObjectSpace.each_object(Module){|e| p e}'. The code already uses defined? to check if the constants actually exist.

In terms of the Date issue, it's only handled on ruby 2.0 and 2.1, and that's because it is available by default in those versions. I guess I could have used --disable-gems (it's defined by rubygems in those versions), but then it doesn't really reflect what ruby loads by default.

Anyway, I don't think this is a bug, though it could certainly be implemented differently.

[PikachuEXE]

Might worth mentioning how those lists of constants are obtained in README (ruby -e 'ObjectSpace.each_object(Module){|e| p e}')

[jeremyevans]

Good idea, I've pushed a README update, as well as a Rakefile task that can generate the files for new ruby versions.

[PikachuEXE]

Great :)