fix explicit version matching on FreeBSD

FreeBSD's regular expression library does not like the pattern used
for matching explicit version strings in `nvm_ls`.  Change the
pattern to something more specific that works on FreeBSD.

nvm.sh

@@ -93,7 +93,7 @@ nvm_ls() {
     return
   fi
   # If it looks like an explicit version, don't do anything funny
-  if [ `expr "$PATTERN" : "v.*.\?.*.\?.*$"` != 0 ]; then
+  if [ `expr "$PATTERN" : "v[[:digit:]]*\.[[:digit:]]*\.[[:digit:]]*$"` != 0 ]; then
     VERSIONS="$PATTERN"
   else
     VERSIONS=`find "$NVM_DIR/" -maxdepth 1 -type d -name "v$PATTERN*" -exec basename '{}' ';' \