reduce fraction of progress bar for verification

Reduce the fraction of the progress bar used for package verification
from 50% to 25%:
  - verification is faster than before due to sha1 improvements
  - in eclair we're now verifying the compressed data rather than
    decompressing it
  - incremental packages (which is what most installs use) write more
    data than is contained in the package.
diff --git a/common.h b/common.h
index 6761159..d962a0a 100644
--- a/common.h
+++ b/common.h
@@ -68,7 +68,7 @@
 
 // Default allocation of progress bar segments to operations
 static const int VERIFICATION_PROGRESS_TIME = 60;
-static const float VERIFICATION_PROGRESS_FRACTION = 0.5;
+static const float VERIFICATION_PROGRESS_FRACTION = 0.25;
 static const float DEFAULT_FILES_PROGRESS_FRACTION = 0.4;
 static const float DEFAULT_IMAGE_PROGRESS_FRACTION = 0.1;