install: Mojave is being released

[fxcoudert]

macOS 10.14 Mojave will be released on September 24th:

• Add support for Mojave.
• Drop support for El Capitan (unsupported as of August 2018)

[fxcoudert]

Recording the current macOS usage stats:

install events in the last 10 days
================================================================================
 1 | macOS High Sierra (10.13)                             | 3,855,965 |  80.54%
 2 | macOS Sierra (10.12)                                  |   510,119 |  10.65%
 3 | OS X El Capitan (10.11)                               |   185,683 |   3.88%
 4 | macOS Mojave (10.14)                                  |   177,059 |   3.70%
 5 | OS X Yosemite (10.10)                                 |    44,276 |   0.92%
 6 | OS X Mavericks (10.9)                                 |    13,716 |   0.29%
 7 | Mac OS X Lion (10.7)                                  |       415 |   0.01%
 8 | macOS (10.15)                                         |       275 |   0.01%
 9 | OS X Mountain Lion (10.8)                             |       225 |   0.00%
10 | Mac OS X Snow Leopard (10.6)                          |       101 |   0.00%
11 | 7                                                     |        10 |   0.00%
================================================================================
Total                                                      | 4,787,844 |    100%
================================================================================

[fxcoudert]

@MikeMcQuaid ping for review, and merge (can't do it myself)

[fxcoudert]

Thanks @JCount!