fix bookdown2mkdocs dep conflict

.travis.yml

@@ -26,10 +26,10 @@ matrix:
       env:
         - DEPS=locked
         - TEST_COVERAGE=true
-        - EXECUTE_DOC_CHECK=true
     - php: 5.6
       env:
         - DEPS=latest
+        - EXECUTE_DOC_CHECK=true
     - php: 7
       env:
         - DEPS=lowest
@@ -61,11 +61,10 @@ before_install:
   - if [[ $TRAVIS_PHP_VERSION != "hhvm" && $TEST_COVERAGE != 'true' ]]; then phpenv config-rm xdebug.ini ; fi
 
 install:
-
+  - if [[ $TEST_COVERAGE == 'true' ]]; then travis_retry composer require --dev $COMPOSER_ARGS satooshi/php-coveralls:^1.0 ; fi
+  - if [[ $EXECUTE_DOC_CHECK == 'true' ]]; then travis_retry composer require --dev --no-update $COMPOSER_ARGS phly/bookdown2mkdocs ; fi
   - if [[ $DEPS == 'latest' ]]; then travis_retry composer update $COMPOSER_ARGS ; fi
   - if [[ $DEPS == 'lowest' ]]; then travis_retry composer update --prefer-lowest --prefer-stable $COMPOSER_ARGS ; fi
-  - if [[ $TEST_COVERAGE == 'true' ]]; then travis_retry composer require --dev $COMPOSER_ARGS satooshi/php-coveralls:^1.0 ; fi
-  - if [[ $EXECUTE_DOC_CHECK == 'true' ]]; then travis_retry composer require --dev $COMPOSER_ARGS phly/bookdown2mkdocs ; fi
   - travis_retry composer install $COMPOSER_ARGS
   - composer show