Update to bblfsh client v3, use UASTv2, add option to choose UAST mode

[carlosms]

• gitbase 0.16 made semantic UAST the new default. Previously the format was annotated. We should also switch to semantic for consistency.
• Update to bblfsh/client-go v3, following the gitbase 0.18 beta changes Update uast-viewer and bblfsh/client-go to work with gitbase 0.18 #279
  • Update Dockerfile dependencies Update dockerfile dependencies for bblfsh client-go v3 #282
  • Update Travis dependencies Update travis dependencies for bblfsh/client-go.v3 #284
• Use newer uast-viewer release with support for UASTv2 Update uast-viewer and bblfsh/client-go to work with gitbase 0.18 #279
• Add an option to choose the UAST mode (semantic, annotated, native). UAST mode selector #283

[carlosms]

@ricardobaeta, should we have the same mode selector as in bblfsh web, bblfsh/web#165?

[carlosms]

Will leave open until we merge the gitbase-0.18 branch into master.