[#94] Fix major version test against 'beta'

From version 17, we took a different approach to download the
documentation. The test has to be done against the numeric part of the
major version, that could include 'beta[0-9]' as in '18beta1'.

Close #94

Author: fluca1978

bin/pgenv

@@ -1401,7 +1401,8 @@ case $1 in
             # due to a missing dependency (docbooks-xsl).
             # Download the docs tarball separately and
             # install it after build.
-            if [[ ${v%.*} -ge 17 ]]; then
+	    major=${v//beta[0-9]/}
+            if [[ ${major%.*} -ge 17 ]]; then
                 PG_DOCS_TARBALL="postgresql-$v-docs.tar.gz"
                 pgenv_debug "Downloading docs tarball"
 
@@ -1462,7 +1463,8 @@ EOF
             # dependency for docs since 17.x and above, extracted
             # docs will be placed into the source directory,
             # as if they were built.
-            if [[ ${v%.*} -ge 17 ]]; then
+	    major=${v//beta[0-9]/}
+            if [[ ${major%.*} -ge 17 ]]; then
                 pgenv_debug "Unpacking docs tarball"
                 $PGENV_TAR zxf "$PG_DOCS_TARBALL"
             fi