remove javaslang dependency

[jknack]

Jooby depends on two general-purpose/collection-like libraries: javaslang and Guava. Ideally will like to remove Guava dependency but isn't possible due Guice depends on it.

Reasons:

• reduce memory footprint required by javaslang classes
• the name change doesn't help and we can't easily upgrade to latest javaslang (vavr)

Users of javaslang can easily add the dependency to their project.