Drop testing for 1.2.1

We are using net/http Client.Timeout which was added in 1.3 and doesn't
exist in 1.2. There's no reason to keep backward compatibility with 1.2
since we're building our own binaries and go1.3 is available for those
who want to build their own binaries.

.travis.yml

@@ -1,7 +1,6 @@
 language: go
 
 go:
-  - 1.2.1
   - 1.3
 
 before_install:

docs/contributing.md

@@ -21,13 +21,14 @@ Getting go
 
 I recommend using gvm which is a go version manager. For instructions
 on how to install see
-[gvm page on github](https://github.com/moovweb/gvm)
+[gvm page on github](https://github.com/moovweb/gvm). InfluxDB
+currently works with go 1.3 and newer version only.
 
 After installing gvm you can install and set the default go version by
 running the following:
 
-    gvm install go1.2.1
+    gvm install go1.3
-    gvm use go1.2.1 --default
+    gvm use go1.3 --default
 
 Project structure
 -----------------