Update preghpages/ghpages commands to use recent versions

[vincentfretin]

Description:
Update preghpages/ghpages commands to use recent versions without vulnerabilities.
Before:

39 vulnerabilities (14 moderate, 21 high, 4 critical)

After:

24 vulnerabilities (9 moderate, 13 high, 2 critical)

I tested npm run preghpages before and after, there is no change to the gh-pages folder produced.

Similar change also done in aframe-inspector repo: aframevr/aframe-inspector#785

Changes proposed:

• Remove shx dependency that should have been removed when we moved from browserify to webpack
• Replace ghpages -p folder by gh-pages -d folder that does exactly the same thing. ghpages is a non maintained wrapper over gh-pages
• Update replace-in-file package used in the scripts/preghpages.js script