Ignore signing for other branches than master

Created by: 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

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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.

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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