Fixes #1275. Fixes pom dynamic revision conversion

Ivy and pom uses slightly different notation for version range and
dynamic revision.
This change fixes the dynamic revisions involving “+”.
First, when a revision like “1.1+” is found, it will now be treated as
“1.+”.
Next, when it finds a revision like “1+” is found, it will be treated
as “+”.
The conversion of “+” is hardcoded to be “[0,)”.
Based on Ivy's behavior, bringing back approximation

I assumed 1.1+ should be treated as 1.+, but it seems like Ivy treats
it more as “any version that starts with 1.1” including 1.10.
Josh’s original implementation approximates this by making ranges for
multiple digits, 1.1~1.2, 1.10~1.20, etc.
Merge pull request #1537 from sbt/fix/1275

Improves pom dynamic revision conversion