Ignore signing for other branches than master

[jmini]

After changes #287 and merge master -> 3.1.x I got this error:

[INFO] --- maven-gpg-plugin:1.6:sign (sign-artifacts) @ openapi-generator-project ---
gpg: keyblock resource `/home/travis/build/OpenAPITools/openapi-generator/sec.gpg': file open error
gpg: skipped "[secure]": secret key not available
gpg: signing failed: secret key not available

Source: https://api.travis-ci.org/v3/job/391192011/log.txt

---

Input from @jimschubert on Gitter:

Travis doesn't allow unencrypting on non-master branches
you'll need to modify that script to publish without signing, only conditionally.
snapshots don't need to be signed.

---

This PR does the described modification.

I have tried the command from my local computer:

• mvn clean deploy -DskipTests=true -B -U --settings CI/settings.xml
• ./gradlew -PossrhUsername="${SONATYPE_USERNAME}" -PossrhPassword="${SONATYPE_PASSWORD}" uploadArchives --no-daemon

[wing328]

looks good to me

[jmini]

Verified on 3.1.x after a merge: master -> 3.1.x.

Travis CI is OK on 3.1.x, Version 3.1.0-20180612.150521-2 was pushed.