Code fixes

[michalbundyra]

• removed duplicated commands in travis config
• fix contributing guide
• ~updated license to wrap long lines~
• removed unused imports
• removed unused variables
• used single quotation instead of double
• ::class notation instead of strings
• removed 2nd param in Next invocation

[Ocramius]

I changed the target to develop, since this is an improvement. Please rebase.

[michalbundyra]

I changed the target to develop, since this is an improvement. Please rebase.

@Ocramius rebased!

[weierophinney]

Thanks, @webimpress