Use Tycho 2.7.5 to ensure that PGP signatures are mirrored

Bundle are using PGP now for ensuring their integrity instead of the
older MD5 and SHA1 checksums in p2. Upgrade to Tycho 2.7.5 that mirrors
already existing PGP signatures for external 3rd party bundles.

releng/org.eclipse.rap.tools.build/pom.xml

@@ -22,7 +22,7 @@
   <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <maven.build.timestamp.format>yyyyMMdd-HHmm</maven.build.timestamp.format>
-    <tycho-version>2.1.0</tycho-version>
+    <tycho-version>2.7.5</tycho-version>
     <signing-plugin-version>1.3.2</signing-plugin-version>
     <tycho.scmUrl>scm:git:https://github.com/eclipse-rap/org.eclipse.rap.tools</tycho.scmUrl>
     <!-- disabled due to bug 393977