Create auto update mechanism for development docs

This patch intends to automatically deploy changes to http://docs.sympy.org/dev/index.html.
This will happen only when a PR gets merged which is basically when a new commit is added to ```master```.

Since I do not have push access, I couldn't complete this. I've tested it on my fork though.

**Access token is required to get it working**

Procedure to get the access token:

Step 1:
```
$ curl -X POST -u <github_username> -H "Content-Type: application/json" -d "{\"scopes\":[\"public_repo\"],\"note\":\"token for pushing from travis\"}" https://api.github.com/authorizations
```
It'll give you a JSON response having a key called "token".

Step 2:
```
$ gem install travis
```
Step 3:

Put the token which you've got from step 1.
```
$ travis encrypt -r sympy/sympy GH_TOKEN=<token> env.global
```
This will give you an access token("secure"). This helps in creating an environment variable named GH_TOKEN while building.

Replace ```<access_token>``` in L8 in ```.travis.yml``` with the actual access token.