Removes composer self-update from .travis.yml, changes update to install

composer self-update has been causing tons of errors in my builds, so
I'm scrapping it.

Author: jeremykendall

.travis.yml

@@ -9,6 +9,5 @@ php:
 script: phpunit
 
 before_script:
-  - composer self-update
-  - composer update
+  - composer install
   - ./bin/pdp-psl