Fix cronjob adding/deletion

Fixes #79.

Be more explicit about how the commands for modifying the crontab should
be piped and switch to fgrep since no regular expressions are necessary
to remove the crontab entry.

subcommands/cron-job

@@ -9,15 +9,15 @@ letsencrypt_cron_job_add () {
   #shellcheck disable=SC2034
   declare desc="Add auto-renew cronjob to dokku user's crontab"
 
-  (crontab -l || true | grep -v "$CRON_CMD" || true; echo "$CRON_JOB") | crontab -
+  ((crontab -l || true) | (fgrep -v "$CRON_CMD" || true); echo "$CRON_JOB") | crontab -
   dokku_log_info1 "Added cron job to dokku's crontab."
 }
 
 letsencrypt_cron_job_remove () {
   #shellcheck disable=SC2034
   declare desc="Remove auto-renew cronjob from dokku user's crontab"
 
-  crontab -l || true | grep -v "$CRON_CMD" || true | crontab -
+  (crontab -l || true) | (fgrep -v "$CRON_CMD" || true) | crontab -
   dokku_log_info1 "Removed cron job from dokku's crontab."
 }