Feature/new travis -- with a proto v2/v3 matrix

[eddelbuettel]

I switched to my maintained fork of the old r-travis setup (personal preference, mostly).

I also turned on a simple 'v2 versus v3' matrix. I did setup builds of ProtoBuf 3.0.0 via Launchpad yesterday -- a bit of hack as I removed gmock (build complications) and Java (we don't need it). I haven't really tested these packages but they should provide genuine ProtoBuf 3.0.0 for us.

So once the PR is in I'll a PR for the keywords

[eddelbuettel]

And this now works correctly (after I corrected a snafu in the Launchpad-generated packages). See eg this build log which has

[...]
* checking for missing documentation entries ... WARNING
[libprotobuf WARNING google/protobuf/compiler/parser.cc:547] No syntax specified for the proto file: /home/travis/build/eddelbuettel/rprotobuf/RProtoBuf.Rcheck/RProtoBuf/proto/addressbook.proto. Please use 'syntax = "proto2";' or 'syntax = "proto3";' to specify a syntax version. (Defaulted to proto2 syntax.)
[libprotobuf WARNING google/protobuf/compiler/parser.cc:547] No syntax specified for the proto file: /home/travis/build/eddelbuettel/rprotobuf/RProtoBuf.Rcheck/RProtoBuf/proto/helloworld.proto. Please use 'syntax = "proto2";' or 'syntax = "proto3";' to specify a syntax version. (Defaulted to proto2 syntax.)
[libprotobuf WARNING google/protobuf/compiler/parser.cc:547] No syntax specified for the proto file: /home/travis/build/eddelbuettel/rprotobuf/RProtoBuf.Rcheck/RProtoBuf/proto/rexp.proto. Please use 'syntax = "proto2";' or 'syntax = "proto3";' to specify a syntax version. (Defaulted to proto2 syntax.)
All user-level objects in a package should have documentation entries.
See chapter ‘Writing R documentation files’ in the ‘Writing R
Extensions’ manual.
[...]

[eddelbuettel]

No objections? May merge this by end of day.