Use pdf-reader gem to count pdf pages.

Further to https://github.com/alphagov/whitehall/pull/1458

Although pdfinfo is slower than imagemagic in some cases, imagemagic is
significantly slower for pdfs with a large number of pages:

```
$ time pdfinfo test.pdf
Title:          Salisbury Plain army basing programme OEA
Keywords:
CreationDate:   Tue May 20 12:46:01 2014
ModDate:        Tue May 20 12:46:01 2014
Tagged:         yes
Pages:          836
Encrypted:      yes (print:yes copy:yes change:no addNotes:no)
Page size:      595 x 842 pts (A4)
File size:      7161312 bytes
Optimized:      yes
PDF version:    1.6

real  0m0.012s
user  0m0.004s
sys 0m0.007s

$ time identify -format %n test.pdf
836

real  1m23.043s
user  1m21.184s
sys 0m1.810s
```

The pdf-reader gem is near instant for both this example, and the one
used in the previous PR.