Drop support for Go 1.3, 1,4. Support Go 1.7+

Go 1.7 moved the `golang.org/x/net/context` package into the standard
library as `context`. Code importing `context` is no longer compiling
for Go versions earlier than 1.7. Since 1.7 was released two years ago
and we don't want to sacrifice quality, we decided to drop support for
1.4 and 1.3 and support 1.7+ now.

.travis.yml

@@ -1,6 +1,5 @@
 ---
 language: go
 go:
-  - 1.3
+  - 1.7
-  - 1.4
   - tip