Replace TravisCI build status icon with CircleCI.

And remove .travis.yml file.

.travis.yml

-language: python
-python:
-    - "2.7"
-before_install:
-    - "export DISPLAY=:99"
-    - "sh -e /etc/init.d/xvfb start"
-install:
-    - "pip install -e git://github.com/edx/xblock-sdk.git@22c1b2f173919bef22f2d9d9295ec5396d02dffd#egg=xblock-sdk"
-    - "pip install -r requirements.txt"
-    - "pip install -r $VIRTUAL_ENV/src/xblock-sdk/requirements/base.txt"
-    - "pip install -r $VIRTUAL_ENV/src/xblock-sdk/requirements/test.txt"
-    - "pip uninstall -y xblock-problem-builder && python setup.py sdist && pip install dist/xblock-problem-builder-2.0.tar.gz"
-    - "pip install -r test_requirements.txt"
-    - "mkdir var"
-script:
-    - pep8 problem_builder --max-line-length=120
-    - pylint problem_builder --disable=all --enable=function-redefined,undefined-variable,unused-variable
-    - python run_tests.py --with-coverage --cover-package=problem_builder
-notifications:
-  email: false
-addons:
-  firefox: "36.0"

README.md

 Problem Builder and Step Builder
 --------------------------------
 
-[![Build Status](https://travis-ci.org/open-craft/problem-builder.svg?branch=master)](https://travis-ci.org/open-craft/problem-builder)
+[![Circle CI](https://circleci.com/gh/open-craft/problem-builder.svg?style=svg)](https://circleci.com/gh/open-craft/problem-builder)
 
 This repository provides two XBlocks: Problem Builder and Step Builder.