Disable testing against Go 1.3 ~ 1.6 to improve tests

In preparation for executing on #15 (already opened in #17) we need to disable
testing against older versions of the Go toolchain. There are a few reasons for
this, the largest being they don't support subtests. These versions, including
Go 1.7 and 1.8, are EOL by the Go authors so there's also lack of upstream
support for these toolchains.

This was discussed a bit in #18 and on [Slack](https://gophers.slack.com/archives/CBP4N9BEU/p1531704009000014).

Fixes #18

Signed-off-by: Tim Heckman <t@heckman.io>

.travis.yml

@@ -1,10 +1,6 @@
 language: go
 sudo: false
 go:
-    - 1.3
-    - 1.4
-    - 1.5
-    - 1.6
     - 1.7
     - 1.8
     - 1.9

README.md

@@ -49,9 +49,9 @@ $ go get github.com/gofrs/uuid
 
 ## Requirements
 
-This package is regularly tested against Go 1.3+, due to issues with the testing
-libraries on Go 1.2 or less. This package may work perfectly fine with Go 1.2,
-but it is not tested and support for this version is not actively maintained.
+Due to subtests not being supported in older versions of Go, this package is
+only regularly tested against Go 1.7+. This package may work perfectly fine with
+Go 1.2+, but support for these older versions is not actively maintained.
 
 ## Usage