improve Trvis's script env's variables handling

Author: xxlabaza

.travis.yml

@@ -15,8 +15,8 @@ script:
 before_deploy:
   - ./mvnw help:evaluate -N -Dexpression=project.version | grep -v '\['
   - export project_version=$(./mvnw help:evaluate -N -Dexpression=project.version | grep -v '\[')
-  - echo $GPG_SECRET_KEYS | base64 --decode | $GPG_EXECUTABLE --import
-  - echo $GPG_OWNERTRUST | base64 --decode | $GPG_EXECUTABLE --import-ownertrust
+  - echo "${GPG_SECRET_KEYS}" | base64 --decode | "${GPG_EXECUTABLE}" --import
+  - echo "${GPG_OWNERTRUST}" | base64 --decode | "${GPG_EXECUTABLE}" --import-ownertrust
   - sed -i -- "s/\${env.SONATYPE_PASSWORD}/$SONATYPE_PASSWORD/g" .settings.xml
   - ./mvnw --settings .settings.xml deploy -DskipTests=true -Dmaven.javadoc.skip=false -B