Update download script to follow new pattern

Dockerfile

@@ -9,7 +9,7 @@ RUN VERSION=`cat /tmp/VERSION` && \
     chmod a+x /usr/local/bin/* && \
     apt-get update && \
     apt-get install -y --no-install-recommends curl ca-certificates unzip && \
-    curl -L https://github.com/vertcoin/vertcoin/releases/download/$VERSION/vertcoin-$VERSION-linux-64bit.zip --output /tmp/prebuilt.zip && \
+    curl -L https://github.com/vertcoin/vertcoin/releases/download/$VERSION/vertcoin-v$VERSION-linux-64bit.zip --output /tmp/prebuilt.zip && \
     echo "$(cat /tmp/CHECKSUM)  /tmp/prebuilt.zip" | sha256sum -c && \
     unzip /tmp/prebuilt.zip -d /tmp && \
     mv /tmp/vertcoind /usr/local/bin && \

VERSION

@@ -1 +1 @@
-v0.12.0
+0.12.0