Update format of 4.2.0 upgrade commands

Make Ubuntu and CentOS commands consistently formatted ( $ sudo ... ) for 4.2.0 upgrade instructions.

Signed-off-by: Sebastien Goasguen <runseb@gmail.com>
Update 4.2.0 upgrade instructions

Update 4.2.0 upgrade instructions for Ubuntu and CentOS to correct apt / yum usage.

Signed-off-by: Sebastien Goasguen <runseb@gmail.com>
Update references to 4.2.0

Update references to version 4.2.0 to version 4.2.x.
Update references to versioned repository 4.2.0 to 4.2 (as the repository should be specific to the 4.2 release, not the point-release).

Signed-off-by: Sebastien Goasguen <runseb@gmail.com>
Move source tarball verification up document: This closes #6

The source tarball verification steps can and should be performed independently of the specific 4.2.x to 4.3.0 upgrade steps. Move the source tarball verification steps to the beginning of the document to simplify the 4.2.x upgrade steps and minimise downtime (should these steps be performed in sequence as described).

Signed-off-by: Sebastien Goasguen <runseb@gmail.com>