Remove Travis support

Currently Travis supports only Ubuntu 12.04 linux 2.6 VMs which are
terribly outdated, and trying to track down why it's hanging on Travis
but not on other similar VMs is more trouble than it's worth at the
moment.

Certainly another more appropriate and up to date CI solution should be
explored.

.travis.yml

-language: go
-
-go:
-  - 1.4.1
-  - tip
-
-env: GOMAXPROCS=4
-
-before_install:
-  - sudo apt-get install libreadline-dev

README.md

 # Delve
 
-[![Build Status](https://travis-ci.org/derekparker/delve.svg?branch=master)](https://travis-ci.org/derekparker/delve)
 [![GoDoc](https://godoc.org/github.com/derekparker/delve?status.svg)](https://godoc.org/github.com/derekparker/delve)
 
 ### What is Delve?