Improves pom dynamic revision conversion

sbt/sbt@5486daf7006462be13b82d6ae225e816f038c9ff fixed #1275, but it didn't close the issue.
Not realizing that, I originally set out to fix #1275, and mostly did code cleanup and minor enhancement.

- `+` now translates to `[0,)`
- `foo+` now errors.

## notes

- http://ant.apache.org/ivy/history/2.3.0/ivyfile/dependency.html
- http://maven.apache.org/enforcer/enforcer-rules/versionRanges.html

/review @havocp