mbstring is not defined as a platform requirement

[punkstar]

Environment:

• PHP Version: 7.1.11
• ICS Parser Version: 2.1.1
• Linux (Heroku)

Description of the Issue:

When deploying to Heroku I got the following fatal error in the logs:

PHP Fatal error:  Uncaught Error: Call to undefined function ICal\mb_split() in /app/vendor/johngrogg/ics-parser/src/ICal/ICal.php:2026 
Stack trace: 
#0 /app/vendor/johngrogg/ics-parser/src/ICal/ICal.php(2070): ICal\ICal->mb_str_replace('\xC2\x85', Array, 'BEGIN:VCALENDAR') 
#1 /app/vendor/johngrogg/ics-parser/src/ICal/ICal.php(317): ICal\ICal->cleanData('BEGIN:VCALENDAR') 
#2 /app/vendor/johngrogg/ics-parser/src/ICal/ICal.php(245): ICal\ICal->initLines(Array) 
#3 /app/src/Punkstar/RugbyFeed/FixtureProvider/ICal.php(34): ICal\ICal->__construct(Array) 
#4 /app/src/Punkstar/RugbyFeed/FixtureSet.php(19): Punkstar\RugbyFeed\FixtureProvider\ICal->getFixtures() 
#5 /app/src/Punkstar/RugbyFeed/FixtureSet.php(31): Punkstar\RugbyFeed\FixtureSet->getFixtures() 
#6 /app/src/Punkstar/RugbyFeed/League.php(85): Punkstar\RugbyFeed\FixtureSet->getEventsFromTeam(Object(Punkstar\RugbyFeed\Team)) 
#7 /app/src/Punkstar/RugbyFeedService/Controller/FixtureController.php(94): Punkstar\RugbyFeed\League->getFixturesForTeam(Object(Punkstar\RugbyFeed\Team)) 
#8  in /app/vendor/johngrogg/ics-parser/src/ICal/ICal.php on line 2026 

Steps to Reproduce:

You can reproduce by deploying this commit to an app on Heroku: punkstar/rugby-data-api@56c8dfc.

Recommended Fix:

Be explicit about needing the mbstring installed and enabled. I added it as a requirement in my app to get around the problem: punkstar/rugby-data-api@d6db1b2#diff-b5d0ee8c97c7abd7e3fa29b9a27d1780R4.