inform user about bundler problem #289

Job	Run time
C++ tests on Linux	1m 22s
Apache2 tests on Linux	2m 13s
Apache2 tests on macOS	2m 14s
C++ tests on macOS	3m 47s
Homebrew packaging tests	6m 10s
Nginx tests on Linux	2m 21s
Nginx tests on macOS	2m 23s
Nginx dynamic module tests on Linux	1m 5s
Node.js tests on Linux	16s
Nginx dynamic module tests on macOS	1m 33s
Node.js tests on macOS	27s
Ruby tests on Linux	1m 5s
Source packaging tests	26s
Ruby tests on macOS	1m 6s
Passenger Standalone tests on Linux	3m 59s
Passenger Standalone tests on macOS	3m 55s
	34m 22s

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