Fix Travis CI Build Status badge

Travis CI builds were migrated from travis-ci.org to travis-ci.com.

Point the Build Status badge to the correct url.

Author: sergio valverde

README.md

@@ -8,7 +8,7 @@ executed at runtime in order to be initialized.
 This includes anything requiring heap allocations, like vectors or hash maps,
 as well as anything that requires non-const function calls to be computed.
 
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+[![Travis-CI Status](https://travis-ci.com/rust-lang-nursery/lazy-static.rs.svg?branch=master)](https://travis-ci.com/rust-lang-nursery/lazy-static.rs)
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