Mixed shell test styles

[hpwxf]

In travis-ci shell script, there are two styles for tests:

• One bracket style

if [ "$VALGRIND" = true ]; then 

• Two brackets style

if [[ "$SONAR" == true ]]; then

The first one is POSIX compliant and the second one is more flexible by only support by ks, bash and zsh.

Should we define a reference style?

Notes:

• https://stackoverflow.com/questions/669452/is-double-square-brackets-preferable-over-single-square-brackets-in-ba
• http://mywiki.wooledge.org/BashFAQ/031

[RUrlus]

Going by the stack overflow answer I would argue that the two brackets style is preferred as portability is not a major concern here.

[RUrlus]

Should be fixed in the move to #74