Don't install libt1-dev on travis

nikic · nikic · commit cc6871152b3c · 2018-09-17T13:48:39.000+02:00
Support for libt1 has been removed from ext/gd in PHP 7.0 by 7dcfdbb, so we no longer need to install this package. It is also no longer available on newer Ubuntu versions.
diff --git a/.travis.yml b/.travis.yml
@@ -16,7 +16,6 @@ addons:
       - librecode-dev
       - libsasl2-dev
       - libxpm-dev
-      - libt1-dev
       - libzip-dev
 
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