Fail builds based on codecov results

[jameslamb]

We have had to do a lot of complicated stuff to ensure our unit tests run on Travis but not on CRAN (e.g. #162 ). For this and other reasons, we should configure codecov to fail our builds if test coverage falls below some threshold (I propose setting it to our current coverage minus 2%).

Example of how to do this: https://github.com/apache/incubator-dubbo/pull/1678/files