Update a number of short descriptions to be 100 characters or less to reflect the new Hub length restriction

@fredlf if you have time to glance over these and make sure I've done a decent job, that'd be appreciated (but if not, that's OK too, just let us know)

Here's the relevant character counts after this change:
```console
$ wc -L */README-short.txt | sort -n
  12 hello-world/README-short.txt
  19 busybox/README-short.txt
  24 nginx/README-short.txt
  26 mageia/README-short.txt
  26 ubuntu/README-short.txt
  29 centos/README-short.txt
  29 jenkins/README-short.txt
  29 registry/README-short.txt
  34 debian/README-short.txt
  34 fedora/README-short.txt
  59 cirros/README-short.txt
  71 opensuse/README-short.txt
  72 scratch/README-short.txt
  76 crux/README-short.txt
  80 golang/README-short.txt
  81 clojure/README-short.txt
  81 perl/README-short.txt
  81 rails/README-short.txt
  84 java/README-short.txt
  84 mongo/README-short.txt
  86 redis/README-short.txt
  89 python/README-short.txt
  92 gcc/README-short.txt
  92 node/README-short.txt
  94 neurodebian/README-short.txt
  94 ubuntu-upstart/README-short.txt
  95 mysql/README-short.txt
  96 buildpack-deps/README-short.txt
  96 hylang/README-short.txt
  96 jruby/README-short.txt
  97 docker-dev/README-short.txt
  98 ruby/README-short.txt
  99 hipache/README-short.txt
  99 postgres/README-short.txt
  99 wordpress/README-short.txt
 100 php/README-short.txt
 100 ubuntu-debootstrap/README-short.txt
```

I realize we lose quite a bit of information in some places with this change, but it's information that's duplicated in the long description too, so should be generally OK.
While designed for web development, the PHP scripting language also provides general-purpose use.