Fix build dependency
Fix build dependency

icu-devtools got introduced with trusty/jessie. Older OS versions
(with ICU < 5.2) only have libicu-dev.

StephenMcConnel/BloomDesktop,JohnThomson/BloomDesktop,gmartin7/myBloomFork,gmartin7/myBloomFork,gmartin7/myBloomFork,BloomBooks/BloomDesktop,andrew-polk/BloomDesktop,gmartin7/myBloomFork,andrew-polk/BloomDesktop,andrew-polk/BloomDesktop,JohnThomson/BloomDesktop,gmartin7/myBloomFork,StephenMcConnel/BloomDesktop,StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,andrew-polk/BloomDesktop,StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,StephenMcConnel/BloomDesktop,gmartin7/myBloomFork,BloomBooks/BloomDesktop,JohnThomson/BloomDesktop,StephenMcConnel/BloomDesktop,andrew-polk/BloomDesktop,BloomBooks/BloomDesktop,JohnThomson/BloomDesktop,andrew-polk/BloomDesktop,BloomBooks/BloomDesktop,JohnThomson/BloomDesktop