Use Rack::Deflater to reduce size of response.

* New gem: capybara
* Setup test database:
  - config/application.yml.example
  - config/database.yml.example
* Setup capybara feature spec

With one fake course. Testing /courses.

siege -t30s -c 10 'http://127.0.0.1:3000/courses'

before:

Transactions:            297 hits
Availability:         100.00 %
Elapsed time:          29.77 secs
Data transferred:         2.60 MB
Response time:            0.46 secs
Transaction rate:         9.98 trans/sec
Throughput:           0.09 MB/sec
Concurrency:            4.61
Successful transactions:         297
Failed transactions:             0
Longest transaction:          0.81
Shortest transaction:         0.10

after:

Transactions:            289 hits
Availability:         100.00 %
Elapsed time:          29.20 secs
Data transferred:         0.79 MB
Response time:            0.54 secs
Transaction rate:         9.90 trans/sec
Throughput:           0.03 MB/sec
Concurrency:            5.38
Successful transactions:         289
Failed transactions:             0
Longest transaction:          1.54
Shortest transaction:         0.10

siege -t10s -c 10 'http://localhost:3000/assets/application.css'

before:

Transactions:            183 hits
Availability:         100.00 %
Elapsed time:           9.63 secs
Data transferred:        15.45 MB
Response time:            0.03 secs
Transaction rate:        19.00 trans/sec
Throughput:           1.60 MB/sec
Concurrency:            0.54
Successful transactions:         183
Failed transactions:             0
Longest transaction:          0.20
Shortest transaction:         0.01

after:

Transactions:            195 hits
Availability:         100.00 %
Elapsed time:           9.90 secs
Data transferred:         1.54 MB
Response time:            0.03 secs
Transaction rate:        19.70 trans/sec
Throughput:           0.16 MB/sec
Concurrency:            0.67
Successful transactions:         195
Failed transactions:             0
Longest transaction:          0.22
Shortest transaction:         0.01

From the figures above we could see data transferred drastically reduced (Data transferred).
Also not we were a bit slower (transaction rate).