build: Add build dependency check for 'gpg'

buildscripts/checkdeps.sh

@@ -21,6 +21,7 @@ _init() {
 
     ## Minimum required versions for build dependencies
     GIT_VERSION="1.0"
+    GPG_VERSION="1.3"
     GO_VERSION="1.5.1"
     OSX_VERSION="10.8"
     UNAME=$(uname -sm)
@@ -180,6 +181,11 @@ check_deps() {
     if [ $? -ge 2 ]; then
         MISSING="${MISSING} git"
     fi
+
+    check_version "$(env gpg --version 2>/dev/null | sed -e 's/^.* \([0-9.\].*\).*$/\1/' -e 's/^\([0-9.\]*\).*/\1/g' | head -1)" "${GPG_VERSION}"
+    if [ $? -ge 2 ]; then
+        MISSING="${MISSING} gpg"
+    fi
 }
 
 main() {