Replace MD5 hash with expected SHA1 hash.

It looks like the MD5 hash of the file has been used here instead of the SHA1 hash that Homebrew expects, which results in an error:

```
$ boxen --debug
# … time passes ...
Debug: Executing 'brew info boxen/brews/jmeter'
Debug: Executing 'brew boxen-install boxen/brews/jmeter'
Error: Could not update: Execution of 'brew boxen-install boxen/brews/jmeter' returned 1: ==> Downloading http://mirror.tcpdiag.net/apache//jmeter/binaries/apache-jmeter-2.11.tgz
Error: SHA1 mismatch
Expected: f3f853c8f79734580a199efd7a2f0a11
Actual: e9b24f8b5f34565831aafcb046e72bdfa9537386
Archive: /Library/Caches/Homebrew/jmeter-2.11-boxen1.tgz
(To retry an incomplete download, remove the file above.)
# ...
```

Diagnosing:
```
$ md5 /Library/Caches/Homebrew/jmeter-2.11-boxen1.tgz
MD5 (/Library/Caches/Homebrew/jmeter-2.11-boxen1.tgz) = f3f853c8f79734580a199efd7a2f0a11
$ shasum /Library/Caches/Homebrew/jmeter-2.11-boxen1.tgz
e9b24f8b5f34565831aafcb046e72bdfa9537386  /Library/Caches/Homebrew/jmeter-2.11-boxen1.tgz
```