Update .gitignore: add vendor and composer.lock  

If these files are not ignored by default - then after "git clone" and 
"composer update", "git status" looks like this:
 
```
$ git status
# On branch develop
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#       composer.lock
#       vendor/andrewsville/
#       vendor/autoload.php
#       vendor/composer/
#       vendor/dg/
#       vendor/kukulich/
#       vendor/nette/
nothing added to commit but untracked files present (use "git add" to track)
```

which is bad UX ;)