Changing SagaAlgorithmProvider.py will add SAGA to the processing menu, but may not work, depending on your setup.
A lot of the SAGA command line options changed between 2.2 and 2.3, so some of the algorithms won't work. Processing thinks it's calling 2.3, and sets the command line options accordingly. SAGA 2.2 then complains about unknown parameters.
I'm using a Mac, with the KyngChaos build of QGIS (2.18.13, Nov 2017)
I got SAGA 2.3 to work by installing the SAGA LTS using homebrew...
brew install saga-gis-lts
shows it's 2.3.2
shows saga_cmd lives in /usr/local/bin
When processing starts up, it looks in the QGIS install folder for SAGA. The KyngChaos build includes saga 2.2, so Processing will choose that, rather than 2.3.2.
To get around that, I edited the SagaUtils.py, commenting out and replacing the line 'testfolder='
folder = None
#testfolder = os.path.join(QgsApplication.prefixPath(), 'bin')
testfolder = '/usr/local/bin'
. . .
Restarted QGIS, and SAGA 2.3 is available :-)