2.1.55: Remove the generation by direct calling of asciidoctor

In order to ensure repeatable book builds and cut down any issues
concerning rendering, the build is only ensured through the
installation of the required versions with bundle and the run of the
Rakefile.

This closes #1039

2.1.54: Remove reference to master for contributors'list

The clone made by travis doesn't seem to get the references.

2.1.53: Merge pull request #1037 from nicktime/patch-1

Fix bzr fastimport plugin install guide

2.1.52: Merge pull request #1038 from notlmn/grammar

add omitted word

2.1.51: Merge pull request #1035 from eyherabh/patch-2

Number consistency

2.1.50: Merge pull request #1034 from eyherabh/patch-1

Replaced "except for by" with "except by"

2.1.49: Merge pull request #1031 from progit/authors

Add authors and publication date to the book

2.1.48: Merge pull request #1029 from lroellin/patch-1

Fix small typo

2.1.47: Merge pull request #1027 from frej/update-hg-fast-export-info

Update Ch9 section about Mercurial imports to match current hg-fast-export release

2.1.46: Merge pull request #1015 from fdmarcin/master

Update installing.asc