Support for ruby 2.5

Changes:

• support for ruby 2.5 #16
• properly use title from Daru::DataFrame #11

v0.3.4 (minor)

• fix issue related to compatibility with modern version of Daru

Minor update

• update hamster dependency up to 3.0

v0.3.2 Minor fixes and changes

• fixed issues with Daru containers (DateTime indexes)
• some development fixes and improvements (testing on ubuntu and osx in one branch, small refactorings, readme update etc)