Fix libxslt/xslt-config error on fresh Docker Compose environment

[jakejarvis]

Fixes [insert bug/issue number]

Hello! Attempting to start a fresh environment with docker-compose up in the develop branch threw the following error in the docker-php-ext-install step:

configure: error: xslt-config not found. Please reinstall the libxslt >= 1.1.0 distribution
ERROR: Service 'drupal' failed to build: The command '/bin/sh -c apt-get update &&     apt-get install -y --no-install-recommends         git zip unzip bzip2 libbz2-dev rsync nodejs         libgd-dev mysql-client openssh-client vim         python python-dev libxslt1.1 &&     docker-php-ext-configure gd --with-freetype-dir=/usr/include/ --with-jpeg-dir=/usr/include/ &&     docker-php-ext-install pdo_mysql gd bz2 xsl &&     pecl install xdebug-2.6.0' returned a non-zero code: 1

As I'm brand new to the codebase, I can't be sure using libxslt-dev doesn't break anything else – but in my very limited testing, the image and site seem to function as expected. :)

Changes proposed in this pull request:

• Install libxslt-dev instead of libxslt1.1 (https://stackoverflow.com/questions/5178416/libxml-install-error-using-pip)

Add @mentions of the person or team responsible for reviewing proposed changes

[jimafisk]

Hey @jakejarvis thanks for contributing to the project! Your change looks great, I just need to make sure it passes our automated tests before pulling in.

Internally we've been using lando for our local dev recently, but the docker-compose steps from the readme are probably your best bet if you're not syncing to our database. Here's a quick video tutorial in case it's helpful, but it sounds like you're already past that, so nice job!