[travis] Upgrade to Ubuntu 18.04.
[travis] Upgrade to Ubuntu 18.04.
stweil/tesseract,tesseract-ocr/tesseract,stweil/tesseract,UB-Mannheim/tesseract,amitdo/tesseract,amitdo/tesseract,tesseract-ocr/tesseract,amitdo/tesseract,tesseract-ocr/tesseract,amitdo/tesseract,stweil/tesseract,tesseract-ocr/tesseract,UB-Mannheim/tesseract,UB-Mannheim/tesseract,UB-Mannheim/tesseract,stweil/tesseract,stweil/tesseract,tesseract-ocr/tesseract,amitdo/tesseract,UB-Mannheim/tesseract