Use 'gpg' as opposed to explicit 'gpg2'

Travis was failing due to being unable to find 'gpg2' and gpg is already
an alias for gpg2 in the base image. Thanks to zakame++ for
verification.

Dockerfile

@@ -25,8 +25,8 @@ RUN buildDeps=' \
     \
     && curl -fsSL ${url}.asc -o ${tmpdir}/rakudo.tar.gz.asc \
     && curl -fsSL $url -o ${tmpdir}/rakudo.tar.gz \
-    && gpg2 --keyserver $keyserver --recv-keys $keyfp \
-    && gpg2 --batch --verify ${tmpdir}/rakudo.tar.gz.asc ${tmpdir}/rakudo.tar.gz \
+    && gpg --keyserver $keyserver --recv-keys $keyfp \
+    && gpg --batch --verify ${tmpdir}/rakudo.tar.gz.asc ${tmpdir}/rakudo.tar.gz \
     \
     && tar xzf ${tmpdir}/rakudo.tar.gz --strip-components=1 -C ${tmpdir}/rakudo \
     && ( \