Explicitly specify behat version in composer.json

After freshly cloning the example repository, composer update was failing with the following:

>composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)                           
Your requirements could not be resolved to an installable set of packages.

>  Problem 1
    - zodyac/behat-extensible-html-formatter dev-master requires behat/behat ~2.4.6 -> satisfiable by behat/behat[v2.4.6].
    - zodyac/behat-extensible-html-formatter v0.1.0 requires behat/behat ~2.4.6 -> satisfiable by behat/behat[v2.4.6].
    - zodyac/behat-extensible-html-formatter v0.1.1 requires behat/behat ~2.4.6 -> satisfiable by behat/behat[v2.4.6].
    - Conclusion: don't install behat/behat v2.4.6
    - Installation request for zodyac/behat-extensible-html-formatter * -> satisfiable by zodyac/behat-extensible-html-formatter[dev-master, v0.1.0, v0.1.1].

Explicitly specifying the version of Behat to use resolves this issue and allows composer to install dependencies.