Stop testing with '--prefer-lowest'

This solve issues with PHPUnit and Xdebug 3.0 for PHP 7.3 and 7.4.
mockery has basically no dependencies so the part of the matrix testing
with '--prefer-lowest' brings little/no value.

.travis.yml

@@ -3,49 +3,12 @@ language: php
 matrix:
   include:
     - php: 5.6
-      env:
-        - DEPS=lowest
-    - php: 5.6
-      env:
-        - DEPS=latest
-    - php: 7.0
-      env:
-        - DEPS=lowest
     - php: 7.0
-      env:
-        - DEPS=latest
-    - php: 7.1
-      env:
-        - DEPS=lowest
     - php: 7.1
-      env:
-        - DEPS=latest
     - php: 7.2
-      env:
-        - DEPS=lowest
-    - php: 7.2
-      env:
-        - DEPS=latest
-    - php: 7.3
-      env:
-        - DEPS=lowest
     - php: 7.3
-      env:
-        - DEPS=latest
-    - php: 7.4
-      env:
-        - DEPS=lowest
     - php: 7.4
-      env:
-        - DEPS=latest
-    - php: nightly
-      env:
-        - DEPS=lowest
-        - COMPOSER_FLAGS="--ignore-platform-reqs"
-    - php: nightly
-      env:
-        - DEPS=latest
-        - COMPOSER_FLAGS="--ignore-platform-reqs"
+    - php: 8.0
 
 before_install:
   # Install extensions for PHP 5.x series. 7.x includes them by default.
@@ -62,8 +25,7 @@ before_install:
     fi
 
 install:
-  - if [[ $DEPS == 'latest' ]]; then travis_retry composer update --no-interaction $COMPOSER_FLAGS ; fi
-  - if [[ $DEPS == 'lowest' ]]; then travis_retry composer update --prefer-lowest --prefer-stable --no-interaction $COMPOSER_FLAGS ; fi
+  - travis_retry composer update --no-interaction
 
 script:
 - |
@@ -99,4 +61,3 @@ deploy:
   on:
     branch: master
     php: '7.1'
-    condition: $DEPS = latest