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.

bump_version.sh

@@ -11,6 +11,9 @@ VERSION_FILE=src/example/_version.py
 HELP_INFORMATION="bump_version.sh (show|major|minor|patch|prerelease|build|finalize)"
 
 old_version=$(sed -n "s/^__version__ = \"\(.*\)\"$/\1/p" $VERSION_FILE)
+# Comment out periods so they are interpreted as periods and don't
+# just match any character
+old_version_regex=${old_version//\./\\\.}
 
 if [ $# -ne 1 ]; then
   echo "$HELP_INFORMATION"
@@ -22,7 +25,7 @@ else
       # A temp file is used to provide compatability with macOS development
       # as a result of macOS using the BSD version of sed
       tmp_file=/tmp/version.$$
-      sed "s/$old_version/$new_version/" $VERSION_FILE > $tmp_file
+      sed "s/$old_version_regex/$new_version/" $VERSION_FILE > $tmp_file
       mv $tmp_file $VERSION_FILE
       git add $VERSION_FILE
       git commit -m"Bump version from $old_version to $new_version"
@@ -34,10 +37,10 @@ else
       # A temp file is used to provide compatability with macOS development
       # as a result of macOS using the BSD version of sed
       tmp_file=/tmp/version.$$
-      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
       git add $VERSION_FILE
-      git commit -m"Bump version from $old_version to $new_version"
+      git commit -m"Finalize version from $old_version to $new_version"
       git push
       ;;
     show)