Enable use of jline.terminal system property even when TERM=dumb

I'd like to use the jline.terminal system property to set a default terminal. 
However this doesn't currently work when TERM=dumb in the environment.

This pull request fixes that problem.

I ran in to this issue while writing [some automated tests for Grails 3 command line shell which uses jline2](https://github.com/grails/grails-core/blob/22132660832cbd5e4a2574376dcc76b104652cbf/grails-shell/src/test/groovy/org/grails/cli/GrailsCliSpec.groovy#L250-L265). I'm using [ExpectIt](https://github.com/Alexey1Gavrilov/ExpectIt) to test shell interactions and I have to use a custom Terminal class for the tests. We have TERM=dumb environment set for gradle builds that run in Travis and that's why we hit the problem.