Fixed failed download: Bumped Tomcat7 to 7.0.55

Fix for:

```bash
$ brew install tomcat7
==> Downloading http://www.apache.org/dyn/closer.cgi?path=tomcat/tomcat-7/v7.0.54/bin/apache-tomcat-7.0.54.tar.gz
==> Best Mirror http://www.eng.lsu.edu/mirrors/apache/tomcat/tomcat-7/v7.0.54/bin/apache-tomcat-7.0.54.tar.gz

curl: (22) The requested URL returned error: 404 Not Found
Error: Failed to download resource "tomcat7"
Download failed: http://www.eng.lsu.edu/mirrors/apache/tomcat/tomcat-7/v7.0.54/bin/apache-tomcat-7.0.54.tar.gz
```