Typically, a Spring Boot maintenance release would not move to a new
minor version of a dependency. However there is a security
vulnerability in Groovy [1] and 2.4.4 is the only release which
contains a fix for it.

The commit upgrades to 2.4.4, thereby ensuring that users of Groovy
are not vulnerable by default. Users of Groovy whose applications are
not affected by the vulnerability may choose to downgrade back to
2.3.11 by overriding Spring Boot's dependency management.

Closes gh-3540

[1] http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-3253