license: allow github to detect MIT license (#743)

Resolves #731

github uses https://github.com/licensee/licensee to detect
the license for the project.  Because the project contains a file
named "check_license.sh" it thinks this is a license file.  It is
unable to detect a license for that file and fails.  Here is the
run output:

```
< ./bin/licensee detect ../zap/
License:        NOASSERTION
Matched files:  LICENSE.txt, check_license.sh
LICENSE.txt:
  Content hash:  d64f3bb4282a97b37454b5bb96a8a264a3363dc3
  Attribution:   Copyright (c) 2016-2017 Uber Technologies, Inc.
  Confidence:    100.00%
  Matcher:       Licensee::Matchers::Exact
  License:       MIT
check_license.sh:
  Content hash:  2ca67b08b61b91f2b3c67fe4581ee9e8dc9da2e8
  License:       NOASSERTION
```

If you rename the file to checklicense.sh it won't think that it is a
license file and will correctly advertise zap as a MIT project

```
< ./bin/licensee detect ../zap/
License:        MIT
Matched files:  LICENSE.txt
LICENSE.txt:
  Content hash:  d64f3bb4282a97b37454b5bb96a8a264a3363dc3
  Attribution:   Copyright (c) 2016-2017 Uber Technologies, Inc.
  Confidence:    100.00%
  Matcher:       Licensee::Matchers::Exact
  License:       MIT
```

Makefile

@@ -51,7 +51,7 @@ ifdef SHOULD_LINT
 	@echo "Checking for unresolved FIXMEs..."
 	@git grep -i fixme | grep -v -e vendor -e Makefile | tee -a lint.log
 	@echo "Checking for license headers..."
-	@./check_license.sh | tee -a lint.log
+	@./checklicense.sh | tee -a lint.log
 	@[ ! -s lint.log ]
 else
 	@echo "Skipping linters on" $(GO_VERSION)