Fix overly match-happy sed commands

We saw in cisagov/postfix-docker#47 that the sed commands in the
bump_version.sh script could inadvertently match the CC0 version in
the README.md file.  This change escapes the periods in the version
before passing it on to sed so that they only match periods and not
just any character.

Co-authored-by: Shane Frasier <[email protected]>

bump-version

@@ -25,6 +25,9 @@ END_OF_LINE
 )
 
 old_version=$(< "$VERSION_FILE")
+# Comment out periods so they are interpreted as periods and don't
+# just match any character
+old_version_regex=${old_version//\./\\\.}
 new_version="$old_version"
 
 bump_part=""
@@ -143,9 +146,9 @@ if [ "$with_prerelease" = true ]; then
 fi
 
 tmp_file=/tmp/version.$$
-sed "s/$old_version/$new_version/" $VERSION_FILE > $tmp_file
+sed "s/$old_version_regex/$new_version/" $VERSION_FILE > $tmp_file
 mv $tmp_file $VERSION_FILE
-sed "s/$old_version/$new_version/" $README_FILE > $tmp_file
+sed "s/$old_version_regex/$new_version/" $README_FILE > $tmp_file
 mv $tmp_file $README_FILE
 git add $VERSION_FILE $README_FILE
 git commit --message "$commit_prefix version from $old_version to $new_version"