ci: update go versions (#22)

Updates go versions tested in Travis CI. Now we test with the three most recent versions as well as Go 1.3. We remove tests for all intermediate versions, assuming that the test with the ancient Go 1.3 will be enough.

.travis.yml

@@ -1,15 +1,9 @@
 language: go
 go:
-- 1.x
 - 1.3.x
-- 1.4.x
-- 1.5.x
-- 1.6.x
-- 1.7.x
-- 1.8.x
-- 1.9.x
-- 1.10.x
 - 1.11.x
+- 1.12.x
+- 1.x
 before_install:
 - go get github.com/axw/gocov/gocov
 - go get github.com/mattn/goveralls