Miscellanous fixes in preparation for 1.6.0

1. Bug fix in NumericDiffCostFunction (Thanks to Nicolas Brodu).
2. Minor documentation update in solver.h
3. Version history update.
4. Bump the version and ABI version.

Change-Id: I951574ddd0b2c4c03b9c79ff33eb9bea549071e7
diff --git a/include/ceres/numeric_diff_cost_function.h b/include/ceres/numeric_diff_cost_function.h
index 555bc3d..4c84d4c 100644
--- a/include/ceres/numeric_diff_cost_function.h
+++ b/include/ceres/numeric_diff_cost_function.h
@@ -230,8 +230,8 @@
     if (N5) parameters_reference_copy[5] = parameters_reference_copy[4] + N4;
     if (N6) parameters_reference_copy[6] = parameters_reference_copy[5] + N5;
     if (N7) parameters_reference_copy[7] = parameters_reference_copy[6] + N6;
-    if (N7) parameters_reference_copy[8] = parameters_reference_copy[7] + N7;
-    if (N8) parameters_reference_copy[9] = parameters_reference_copy[8] + N8;
+    if (N8) parameters_reference_copy[8] = parameters_reference_copy[7] + N7;
+    if (N9) parameters_reference_copy[9] = parameters_reference_copy[8] + N8;
 
 #define COPY_PARAMETER_BLOCK(block)                                     \
   if (N ## block) memcpy(parameters_reference_copy[block],              \